During x-ray imaging, x-rays are produced through the generation of electrons by thermionic emission from a cathode, often a tungsten filament, and the acceleration of these electrons within an x-ray tube towards an anode, which causes the emission of x-rays. The emission intensity of the x-rays from the x-ray tube is controlled by the filament current and by the selected voltage potential differential between the anode and the cathode. The selected voltage potential is generally several tens of thousand kilovolts (kV).
There are different types of x-ray imaging known in the art. One type of x-ray imaging, often referred to as radiographic imaging, generally requires a high emission dose from the x-ray tube and is intended for film imaging. Radiographic imaging requires large amounts of x-ray radiation for short periods of exposure time. Another type of x-ray imaging, often referred to as fluoroscopic imaging, generally requires lower emissions from the x-ray tube producing lower amounts of x-ray dose but for longer periods of time. Because of this, fluoroscopic imaging is generally intended for “live” electronic monitoring of the body. This may be done, for example, during a medical procedure when a doctor leads an object through the body and requires continuous imaging of the body and the object in order to properly place the object in the body during the procedure.
Several sub-methods of fluoroscopic imaging have also been used. These may be referred to, in some cases, as continuous (or CW) fluoroscopy, which requires that the x-ray source remain turned on for long periods of time in order to provide “live” electronic monitoring of the body.
With the improvement of imaging techniques, it has been found that short fluoroscopic pulses of x-rays could be detected and held electronically on a monitor and then replaced with a subsequent new image from a further short fluoroscopic pulse of x-rays. This submethod of fluoroscopic imaging, sometimes referred to as pulsed fluoroscopy, has pulses which can vary from a few pulses per second to 30 pulses per second. It is understood that at this rate of pulse fluoroscopy, the intermittent nature of the pulses may not be immediately apparent during the electronic monitoring of the body, or, does not cause a serious degradation to the imaging.
In fluoroscopic imaging, regardless of the specifics of methods used and whether it is continuous or pulsed, the emission from the x-ray tube and the corresponding power level (MA) to the tube is low. This necessitates that the current flowing through the high voltage circuit to the x-ray tube is also low. In these types of situations, when the power supply, also referred to as the generator, stops generating the selected voltage at the termination of an x-ray exposure, whether pulsed fluoroscopy, continuous fluoroscopy or even radiography, a large charge capacitance is left in the components of the imaging system. For example, the system may have a cathode cable extending from the generator to the cathode of the x-ray tube and an anode cable extending from the generator to the anode of the x-ray tube. These cable sets can have a length of typically 50 feet up to 100 feet long. Furthermore, cable sets of sufficient capacity to carry the current and, more importantly, voltage to generate x-rays may have relatively large capacitance between the core and the shield in the cable set, such as in the order of about 50 pico farads (pF) per foot. For cable lengths of 50 to 100 feet, the capacitance of the cable set can be between approximately 2500 pico farads to 5000 pico farads per cable. Using this capacitance and using the simple energy equation of a charged capacitor to emulate the capacitance of the shielded cable, the stored energy in a 50 foot cable can be estimated as:E=½CV2=½(2500×10−12)×625002=4.9 jouleswhen the cable is 50 feet and the voltage at termination is 62.5 kV. The stored capacitive energy will be twice as much if a 100 foot cable is used and there will be an equal amount of energy stored in the anode cable.
Because the x-ray tube during fluoroscopy is generally set to a low emission level for minimal x-ray production, the x-ray tube will not discharge the capacitive energy of the high voltage cables nor the other components in the x-ray system quickly. The fluoroscopic pulsing may then exhibit a “tail” of x-rays that can last into the next pulse. This is illustrated, for instance, by the shaded area 2 in FIG. 1A, which illustrates the potential differential between the anode cable and the cathode cable in conventional x-ray imaging systems. For completeness, FIGS. 1B and 1C, illustrating the voltage differential between the anode cable to ground and the cathode cable to ground, respectively, is also provided.
The “tail” of x-rays is detrimental for several reasons. For instance, the “tail” of x-rays represents excess radiation absorbed dosage to the patient, as well as increased x-ray scatter to others in the vicinity of the patient, including the physician and the assisting staff. Furthermore, the “tail” of x-rays is also detrimental during the x-ray imaging as it generally causes additional detected radiation which is not useful or may have detrimental imaging value because it has a diminishing waveform characteristic. This effect increases exposure time which increases motion artefacts to the displayed image.
Several methods and devices have been used in the past in order to subtract or eliminate the “tail” effect of pulsed fluoroscopic waveforms. One such method involved the use of an x-ray tube which had a third element such as a grid (also referred to as a cathode cup). The grid could be used to turn off the x-rays at high speed by energizing the grid at the appropriate moment. However, grid-type tubes had limited radiographic abilities at higher power levels and at higher voltage thereby limiting grid-type x-ray tubes to lower voltage potentials than non-grid tubes. Another disadvantage of grid-type x-ray tubes is that certain regulatory agencies require a mechanical “flapper” to be added to the x-ray tube port to prevent exposing patients and staff to continuous x-ray should the grid bias be lost. Furthermore, use of the grid-type tubes increased the operating costs of the overall x-ray imaging system because grid-type tubes require a third control line which many systems do not provide and the replacement cost of a grid-type x-ray tube is much higher than non-grid tubes.
Other methods have been considered for eliminating this “tail” effect. For instance, U.S. Pat. No. 5,056,125 to Beland discloses a system having a series of Triac switches connected in series and including discharge resistors and ballasting capacitors. The Triac switches were connected together from both the anode high voltage cable to ground and from the cathode high voltage cable to ground. In addition to having a first connection and a second connection for supporting or conducting the current, the Triac switches also have a third connection, namely a gate, for triggering the switch. The gates in the series of Triac switches were located in the high voltage portion of the apparatus and the Triac switches were activated by a trigger signal which was generated by a low voltage portion of the apparatus. While the Beland device operates relatively well, it suffers from the disadvantage that there is a time lag required for each of the switches to activate gates in the series of switches. Furthermore, there is increased circuitry involved in connecting the three connections of each switch for the number of switches required to support the voltage differential between the cable sets and ground. Furthermore, Beland requires that switches be present to quench the power from both the anode cable and the cathode cable requiring a large number of switches and also requiring that the discharge trigger emanating from the low voltage portion be sent to both the switches connected to the anode cable and the cathode cable.
Another device for discharging a cable set is disclosed in U.S. Pat. No. 5,077,770 to Sammon. The Sammon device utilizes a xenon tube, or a similar high voltage flash tube, or another type of device which has an ionizable material. Sammon discloses that one xenon tube is connected between the anode cable and ground to support the voltage between the anode cable and ground and another xenon tube is connected between the cathode cable and ground to support the voltage between the cathode cable and ground. Sammon discloses that a voltage tickler coil triggers each of the xenon tubes simultaneously in order to ionize the gas in the xenon tube causing the xenon tube to become electrically conductive. While the Sammon device is relatively quick, it suffers from the disadvantage that it requires two xenon tubes which are expensive, must be periodically replaced, and may exhibit changing characteristics over time and use. Also, a relatively large voltage is required to ionize the gas in the xenon tubes increasing the overall cost of operation and the heat generation.